A bus is a set of conductors that serve as a common data transmission path for a set of computer components. For example, a bus may serve as a common data transmission path for a central processing unit (CPU), a hard disc drive storage device, and an external connector that is used for attachment to some type of peripheral device, such as a printer.
If a bus were a perfect transmission line, then high signals (digital ONES) written to the bus would experience a uniform rise. However, the computer components connected to the bus create distorted high signals. Specifically, the computer components connected to the bus constitute lumped capacitive loads and stubbs that can produce a digital ONE signal with several spurious signal transitions before a final peak value is reached. FIG. 1 illustrates a digital ONE signal that is commonly produced on a computer bus. The signal includes an initial rising component 10, an intermediate falling component 12, a flat component 14, a final rising component 16, a final high or peak value 18 (digital ONE), and a signal completion falling component 19. The intermediate falling component 12 and flat component 14 can be characterized as spurious signal transitions. As used herein, the term spurious signal transition refers to any signal transition that erroneously represents a change in signal value. A computer component connected to the bus interprets a spurious signal transition as a change in signal value. Consequently, the computer component processes erroneous data signals.
The problem of spurious signal transitions has been addressed by processing a signal from a bus with a low pass filter and a Schmitt trigger. Unfortunately, this type of circuit cannot distinguish between the initial rising component of the waveform and the final high value of the waveform when there is an intermediate falling component in the waveform.
Thus, it would be highly desirable to provide a filter for signals received from a computer bus. In particular, it would be highly desirable to provide a circuit that does not erroneously respond to spurious transitions in a signal received from a computer bus.